The present invention relates to an attachment or a conversion lens, particularly a lens to be mounted between the principal lens and the image plane in such a manner that the optical axis coincide with each other, in order to increase the focal length of the principal lens.
Until now, it is widely known to mount a conversion lens at the object side or the image side of the photographing lens so as to increase or decrease the focal length while the image plane of the photographing lens is kept constant. Hereby, in the case of the front conversion lens mounted at the object side of the photographing lens, the size of the conversion lens, particularly the diameter of the front lens, is very large and cannot be made small. On the other hand, there is a possibility to realize a small rear conversion lens which is mounted at the image side of the photographing lens.
The rear conversion lens known until now are used for the so-called interchangeable lens camera. There are disclosed for example in Japanese Patent Application Laid-Open Nos. Sho 54-63752, Sho 54-25820, Sho 54-834 and Sho 54-53528. Further, the technique in accordance with which the rear conversion lens is built in the camera body so as to make the whole system compact is disclosed in Japanese Patent Application Laid-Open No. Sho 54-97423. However, the rear conversion lens for the interchangeable lens camera consists of 5 to 7 lenses and, thus, there are problems in the size and the cost. Further, the conversion lens which is built in the camera body consists of 6 lenses, and the size can be compared to that of the principal lens, which cannot be said to be compact.